The invention relates to ultrasonic imaging systems, and more particularly to systems that utilize an acoustic lens to focus ultrasonic energy.
The users of medical ultrasonic transducer probes, hereinafter referred to as sonographers, can obtain images of a region within a body by properly positioning a probe against the body. In order to obtain images having diagnostic value, the sonographer may have to physically manipulate the position of the probe by sliding, rotating, and tilting the probe. One area in particular where this manipulation is more challenging is transesophageal cardiac imaging. During transesophageal cardiac imaging, the sonographer positions a transducer housing at the tip of the probe against the esophagus or stomach of a patient in order to obtain different fields of view of the heart.
Typically for this application, the transducer housing contains a number of acoustic transducer elements, which may be sequentially electrically excited by an ultrasound control and operating system to obtain an image in an object plane that is perpendicular to the transducer housing and the transducer elements. It has been found desirable to rotate the transducer elements contained within the transducer housing independently from the physical manipulation of the housing itself. In combination with the ability to slide, rotate and tilt the transducer housing, the ability to independently rotate the transducer elements within the housing gives the sonographer the ability to obtain an ultrasonic image of any or all object planes orthogonal to the upper surface of the transducer elements at each location to which the housing can be moved.
Because the transducer housing on this type of probe may be placed within the body of the patient being examined, it must be sealed to protect against the ingress of bodily fluids, sterilant solutions, and cleaning solutions. The seal may also prevent moving parts, such as the rotating transducer elements, from contacting the body of the patient.
The acoustic energy radiated from the transducer housing may be focused by mounting an acoustic lens to the transducer elements within the housing. A cylindrical lens, which does not vary in thickness along the azimuthal direction, is sometimes used to provide a fixed focus to the transducer array in the elevation direction.
Devices are known that utilize a cylindrical lens. For example, U.S. Pat. No. 5,127,410 to King et al. includes a compound lens secured on top of a rotatable transducer. The compound lens may have a convex cylindrical lens of a first material mated to a concave lens of a second material. The second material has substantially the same acoustic properties as the body being scanned. The concave portion of the compound lens is shaped so that the upper surface of the compound lens is flat. The flat upper surface of the compound lens is mated to a flat lens assembly that seals an opening in the transducer housing. The opening is sealed, for example by a thin plastic film that is bonded to the housing in the area around the opening. A backing layer is formed to the plastic film and an RFI screen is embedded in the backing layer. A grease layer lubricates the interface between the flat upper surface of the compound lens and the flat lens assembly that seals the transducer housing.
A disadvantage of the King device is that a number of material layers are added to the top of the transducer elements, which can cause degraded performance due to attenuation within the layers or reflections at the material interfaces giving rise to reverberation artifacts in the resultant image. In addition, the concave lens may be difficult to fabricate. Few solid materials possess the acoustic properties required of the concave lens, and those materials that do possess the required properties may not adhere to the convex lens material and may absorb certain grease or oil materials causing the concave lens to expand. Expansion of the concave lens may lead to distortion of the transducer housing's component parts, unreliability, and a reduction in image quality. Furthermore, the added process steps in fabricating and assembling each material layer may decrease yield.
A further disadvantage of the King device is that the compound lens may require more space within the transducer housing than would a simple lens. It is desirable to minimize the size of the transducer housing so that it may be easily inserted by the sonographer and manipulated within the body of the patient without causing excessive patient discomfort.
A further disadvantage of the King device is that the flat lens assembly that seals the opening in the transducer housing may deform or make poor contact with the body of the patient upon application of pressure by the sonographer. It is desirable to maintain acoustic contact with the body of the patient because the presence of any gaps between the transducer housing and the patient will seriously degrade performance.
A still further disadvantage of the King device is that additional process steps are required to seal the opening in the housing and the seal may introduce additional material layers between the transducer elements and the body of the patient. As noted above, it is desirable to minimize the number of material layers between the transducers and the patient and to minimize processing steps in constructing the transducer housing.
Accordingly, it would be desirable to have an improved ultrasonic transducer probe.